Exhaust gases from internal combustion engines comprise a mixture of pollutants including carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter (PM). The NOx component can comprise nitrogen monoxide (NO) and nitrogen dioxide (NO2). The level of these pollutants in exhaust gases from internal combustion engines it is permissible to exhaust to atmosphere is regulated by legislation. Such legislation can be met by engine design, engine management and/or exhaust gas aftertreatment, and typically a combination of all three measures.
A prior art exhaust system primarily for treating diesel exhaust comprises an oxidation catalyst for oxidising NO in the exhaust gas to NO2 and a downstream filter for trapping PM. A process for treating diesel PM that uses this arrangement is described in EP 0341382 or U.S. Pat. No. 4,902,487, both of which are incorporated herein by reference. The process comprises passing an exhaust gas, such as a diesel exhaust gas, including PM and NO unfiltered over an oxidation catalyst to convert the NO to NO2, collecting PM on the filter and combusting the collected PM by reaction with the NO2. This, technology is commercially available as Johnson Matthey's Continuously Regenerating Trap or CRT®. Combustion of the PM in NO2 results in CO and NO, with a potential side-reaction leading to the complete reduction of the NO2 to N2 as described in SAE 890404.
An advantage of this process is that it is possible to combust diesel PM at temperatures of up to 400° C., whereas combustion of diesel PM in oxygen occurs at about 500° C. and above. This is significant since diesel exhaust gas is generally cooler than exhaust gas from gasoline engines and PM would accumulate on the filter causing back-pressure problems in the system if the process relied solely on combustion of PM in oxygen without provision of additional means for increasing the exhaust gas temperature; so-called “active” regeneration regimes.
A problem with the process described in EP 0341382 is that, as exhaust emission legislation has tightened since the publication of that application, legislative bodies have begun to discuss limiting the amount of NO2 it is permissible to exhaust to atmosphere. For example, the California Air Resources Board (CARB) has proposed that a maximum of 20% of tailpipe NOx of the relevant drive cycle is emitted as NO2 (See California's Diesel Risk Reduction Program, September 2000 and Title 13, California Code of Regulations, Chapter 14, section 2706.). NO2 is toxic and can cause headaches, dizziness and nausea in low doses. It also has an objectionable smell. If there is insufficient PM on the filter to react with NO2 generated over the oxidation catalyst or the temperature of the exhaust gas is below a preferred range for combustion of PM in NO2, NO2 can slip past the filter and be undesirably exhausted to atmosphere.
This problem is particularly acute when internal combustion engines are used in confined spaces, such as mines, where vehicles are used to drill for, load, and transport mined material to the surface. Many mining operations generate particulate matter, and so exhaust aftertreatment systems comprising filters for reducing the levels of PM emitted are being considered. Furthermore, explosives used to blast rock to recover a desired ore can generate NO2. Accordingly, it would be an advantage to reduce the exhaust gas emissions of both PM and NO2 to the atmosphere in closed environments to improve the health and safety of miners. Indeed, the US Mine Safety and Health Administration prevents the use of diesel exhaust systems comprising diesel particulate filter systems that increase NO2 emissions.
In selective catalytic reduction (SCR) by hydrocarbons (HC), HC react selectively with NOx, rather than with O2, to form nitrogen, CO2 and water according to equation (1):{HC}+NOx→N2+CO2+H2O   (1)
The competitive, non-selective reaction with oxygen is given by Equation (2):{HC}+O2→CO2+H2O   (2)
Two preferred groups of HC-SCR catalysts to selectively promote the desired reaction (1) for catalysing HC-SCR of NOx (HC-SCR catalysts are also referred to as “lean NOx catalysts” (LNC), “DeNOx catalysts”, “NOx occluding catalysts”, “NOx reducing catalysts” and even “non-selective catalytic reduction catalysts” (because they can catalyse non-selective reactions e.g. Equation (2)). These two preferred groups are platinum on alumina and copper-substituted zeolite such as Cu/ZSM-5.
Pt-based catalysts tend to operate at relatively low temperature (peak activity ˜250° C.) and have a relatively narrow temperature window for HC-SCR activity whereas zeolite-based HC-SCR catalysts have a wider temperature window than Pt-based HC-SCR catalysts and operate at higher temperatures (peak activity ˜400° C.).
One potential solution to this problem is described in EP 0758713, where in one embodiment, an exhaust system comprises an optionally platinum-based oxidation catalyst and a diesel particulate filter (DPF) in the CRT® configuration and a NOx absorbent downstream of the DPF. The NOx absorbent can comprise platinum for oxidising NO to NO2 in lambda >1 exhaust gas compositions, rhodium for reducing NOx to N2 in lambda <1 exhaust gas compositions and at least one substance selected from alkali metals such as potassium and caesium; alkali-earth metals such as barium and calcium; and rare-earth metals such as lanthanum for absorbing the NO2 and storing it as the nitrate. Catalyst compositions comprising platinum, rhodium and a NOx absorbent material are typically called NOx traps.
In a second embodiment of EP 0758713, a NOx reducing catalyst is disposed downstream of the filter for catalysing the reduction of the NOx to N2 using diesel HC fuel and CO. The NOx reducing catalyst can be a zeolite such as ZSM-5 ion exchanged with copper or iron, or mordenite supporting platinum. However, it is clear from EP 0758713 that HC reductant for reducing the NOx is introduced into the exhaust system either by injecting additional fuel during the exhaust cycle or directly into the exhaust passage. In either case, injection is done always upstream of the CRT® oxidation catalyst.
EP 0888816 discloses an exhaust emission control catalyst containing the three metals copper, praseodymium and yttrium, wherein the hydrocarbon: nitrogen oxide mole ratio is within a range of from 0.5 to 30.
EP 0541271 discloses a catalyst system for treating NOx in the exhaust from a lean-burn gasoline-fueled engine, which system comprising a first stage catalyst containing a transition metal-exchanged zeolite (i.e. Cu-ZSM5), and a second stage catalyst, which is a three-way catalyst, for treating the effluent from the first stage catalyst. The engine is controlled such that the ratio of NOx to HC in the exhaust gas is in the range of from 1/3 to 3/1 (i.e. minimum C3H6 of 250 ppm and NOx of 200-400 ppm). Only the performance of the second stage catalyst and the first and second stage catalysts in combination is assessed in the Examples.
WO 03/037507 describes an exhaust system for an internal combustion engine comprising a catalyst, such as a platinum-based catalyst, for oxidising NO to NO2 when the exhaust gas composition is lambda >1; and a filter disposed downstream of the NO oxidation catalyst, i.e. in the CRT® configuration. The filter can comprise an oxidation catalyst such as platinum and/or palladium, rhodium and a NOx absorbent material, such as any of those described in EP 0758713 above. A filter component of this arrangement is described in Japanese patent no. 2722987.